You could replace the cocoa with white chocolate chips or chopped white chocolate. I would reduce the sugar as white chocolate is very sweet. Also, the amount of milk would likely have to be cut back. My first attempt on a white chocolate pie would be to reduce the sugar to 1/4 cup, use 1 cup white chocolate chips and reduce the milk to 2 cups while leaving all the other ingredients the same. If you try it, let me know how it turns out!